Surface layer grain refining hot-shearing method and workpiece obtained by surface layer grain refining hot-shearing

ABSTRACT

Provided is a surface layer grain refining hot-shearing method including: heating and keeping a steel sheet in a temperature range of from Ac3 to 1400° C. to austenitize the steel sheet; subsequently shearing the steel sheet in a state in which the steel sheet is placed on a die; and quenching by rapidly cooling the sheared steel sheet, wherein a start temperature of the shearing is set to be a temperature (° C.) obtained by adding a temperature of from 30° C. to 140° C. to a previously measured Ar3 of the steel sheet.

TECHNICAL FIELD

The present invention relates to a surface layer grain refininghot-shearing method of a steel sheet, which has a carbon content of0.15% or more by mass and is used in automobiles, ships, bridges,construction equipment, various plants, or the like, and a workpieceobtained by surface layer grain refining hot-shearing.

BACKGROUND ART

From the past, a metal material (steel sheet) to be used in automobiles,ships, bridges, construction equipment, various plants, or the like hasbeen often subjected to shearing by a punch and a die. Recently, fromthe viewpoint of safety and weight lightening, various members becomehigh strengthening, and as disclosed in “Press Technology”, Vol. 46, No.7, p. 36-41 (hereinafter, referred to as “Non-Patent Literature 1”), aquenching press is performed in which press forming and heat treatmentare almost simultaneously performed to form a high-strength member.

A general cold-pressed workpiece is subjected to shearing such aspunching and trimming after being subjected to press forming. However,when the quenching-pressed workpiece is subjected to shearing afterbeing subjected to forming, a service life of a shearing tool becomessignificantly shorter due to high hardness of the member. In addition,there is a concern that delayed fracture occurs due to residual stressin a sheared portion. Thus, the quenching-pressed workpiece is oftensubjected to laser cutting rather than the shearing.

However, since the laser cutting requires costs, for example, thefollowing methods have been proposed so far: a method of performing aheat treatment after shearing (for example, see Japanese PatentApplication Laid-Open (JP-A) No. 2009-197253 (hereinafter, referred toas “Patent Literature 1”)); methods of reducing residual stress in asheared portion by simultaneously performing shearing and hot pressingbefore quenching (for example, see JP-A No. 2005-138111 (hereinafter,referred to as “Patent Literature 2”), JP-A No. 2006-104526(hereinafter, referred to as “Patent Literature 3”), and JP-A No.2006-83419 (hereinafter, referred to as “Patent Literature 4”)); amethod of reducing quenching hardness by gradually lowering a coolingrate of a sheared portion (for example, see JP-A No. 2003-328031(hereinafter, referred to as “Patent Literature 5”)); a method ofworking to soften only a shearing scheduled portion by performing localelectric-heating (for example, see “CIRP Annals-ManufacturingTechnology” 57 (2008), p. 321-324 (hereinafter, referred to as“Non-Patent Literature 2”)); and a shearing-related technology forcontrolling structures in a surface layer of a shear plane in ahigh-strength steel sheet to improve delayed fracture resistance (seeJP-A No. 2012-237041 (hereinafter, referred to as “Patent Literature6”)).

SUMMARY OF INVENTION Technical Problem

There are several problems in the methods disclosed in PatentLiteratures 1 to 6 and the method disclosed in Non-Patent Literature 2.According to the method disclosed in Patent Literature 1, since themethod can be used for only a specific material and is used to performshearing on a quenched material, the problem such as deterioration inservice life of the tool is not solved.

According to the methods disclosed in Patent Literatures 2 to 4, theresidual stress in the sheared portion caused by deformation resistanceof the steel sheet can be reduced, but it is not possible to reducethermal stress caused by seizure of the tool and non-uniformity of acontact with a die during quenching and to reduce residual stress causedby transformation of the steel sheet. Therefore, when ductility of thehot-sheared portion is low, the problem such as occurrence of thedelayed fracture is not solved. A method of improving the ductility ofthe hot-sheared portion is not disclosed in Patent Literatures 2 to 4.

According to the method disclosed in Patent Literature 5, it isconsidered that ductility can be improved because the sheared portion ofthe steel sheet is not hardened, but a shearing time becomes longer andthus costs increase as the cooling rate becomes slower. According to themethod disclosed in Non-Patent Literature 2, it is necessary to preparea new die formed with an electric heating apparatus for shearing andthus costs increase.

According to the method disclosed in Patent Literature 6, it has anexcellent effect of improving the delayed fracture resistance, but ashearing start temperature of from 400° C. to 900° C. is definedregardless of a material of a member to be sheared or a cooling rate.For this reason, the shearing may occur at a temperature range(low-temperature side), at which the delayed fracture occurs, dependingon the materials of the member to be sheared or shearing conditions.Conversely, when the shearing is performed at a high temperature morethan necessary such that the delayed fracture does not occur, the amountof thermal expansion becomes larger and a dimensional change becomeslarger at the time of returning to an ambient temperature. As a result,the dimensional error of the workpiece becomes greater. Therefore, in acase in which the shearing temperature is precisely controlled at thelower temperature according to actual shearing conditions, there stillremains a possibility of suppressing the delayed fracture while furtherimproving shearing accuracy of the workpiece.

Patent Literature 6 discloses that the delayed fracture does not occurwhen fine ferrite is present in the surface of a shearing portion.However, for example, in experimental numbers 36 to 40 in which a steelsheet A8 indicated in Table 5 obtained by steel sheet component A8 or A9indicated in Table 1 of Example is used, even when the shearing isperformed at the same shearing temperature and cooling rate under thesame heating conditions and keeping conditions, structures vary and thusthe delayed fracture may occur in some cases. Even when a steel sheet A9indicated in Table 5 is used, the same results were obtained.

In order to solve the above problems, the invention has tasks to preventdelayed fracture occurring in a hot-sheared portion and to improveshearing accuracy of a workpiece without increasing the shearing timeand new steps, and an object thereof is to provide a surface layer grainrefining hot-shearing method and a workpiece obtained by surface layergrain refining hot-shearing, which meets these requirements, for thepurpose of achieving of these tasks.

Solution to Problem

The present inventors have intensively studied on a technique forsolving the above problems. As a result, the inventors found that in acase in which a temperature for staring shearing (shearing starttemperature) is set to an appropriate range based on the amount ofequivalent plastic strain of a surface layer of a sheared portion,delayed fracture does not occur even when high residual stress remainsin the sheared portion.

That is, the amount of equivalent plastic strain of the sheared portionis affected by a temperature during the shearing and a structure beforethe shearing (ferrite or austenite), but a structure after the shearingis differently changed depending on the amount of equivalent plasticstrain of the sheared portion and the shearing temperature. As to howthe structure differs, compositions of the steel sheet, pressingconditions and temperature histories associated with these pressingconditions when pressing is performed before the shearing contributethereto. The inventors found conditions in which even when high residualstress remains in the sheared portion, the dimension accuracy isimproved without an occurrence of the delayed fracture by optimizing theshearing temperature in view of all these factors.

In particular, the inventors confirmed, in a carbon steel for machinestructural use defined in JIS G 4051 having a carbon content of 0.15% ormore by mass or having preferably a carbon content of 0.48% or less bymass in view of cold workability after shear cooling, that the inventionwas applicable to cold-rolled steel sheets of S17C, S25C, S35C, and S45Cdefined in JIS G 4051 when an actually measured Ar3 point isapproximately 500° C. or lower at the time of cooling by leaving.

The invention has been made based on the above findings and the gistthereof is as follows.

A first aspect of the invention is to provide a surface layer grainrefining hot-shearing method including: heating and keeping a steelsheet having a carbon content of 0.15% or more by mass in a temperaturerange of from Ac3 to 1400° C. to austenitize the steel sheet;subsequently shearing the steel sheet in a state in which the steelsheet is placed on a die; and quenching by rapidly cooling the shearedsteel sheet, wherein a start temperature of the shearing is set to be atemperature (° C.) obtained by adding a temperature of from 30° C. to140° C. to a previously measured Ar3 of the steel sheet.

A second aspect of the invention is to provide a surface layer grainrefining hot-shearing method including: heating and keeping a steelsheet having a carbon content of 0.15% or more by mass in a temperaturerange of from Ac3 to 1400° C. to austenitize the steel sheet;subsequently shearing the steel sheet in a state in which the steelsheet is placed on a die; and quenching by rapidly cooling the shearedsteel sheet, wherein a start temperature of the shearing is set to be atemperature (° C.) obtained by adding a value, which is calculated bymultiplying an amount of equivalent plastic strain of a surface layer ina sheared portion by a coefficient from 40 to 60, to a previouslymeasured Ar3 of the steel sheet.

A third aspect of the invention is to provide the surface layer grainrefining hot-shearing method according to second aspect of theinvention, wherein the amount of equivalent plastic strain of thesurface layer in the sheared portion is calculated as an average valueof an amount of equivalent plastic strain of a region in a range of from5% to 20% of a thickness of the steel sheet from a shear plane of thesheared portion to an inside of the steel sheet in a normal direction ofthe shear plane and in a range of from 20% to 50% of the thickness ofthe steel sheet in a thickness direction of the steel sheet from abottom on a burr side of the sheared portion.

A fourth aspect of the invention is to provide the surface layer grainrefining hot-shearing method according to the second or third aspect ofthe invention, wherein the amount of equivalent plastic strain of thesurface layer in the sheared portion is calculated by a numericalsimulation that is performed based on a stress-strain diagram at a steelsheet temperature of from 500° C. to 800° C.

A fifth aspect of the invention is to provide the surface layer grainrefining hot-shearing method according to any one of the second aspectto the fourth aspect of the invention, wherein the amount of equivalentplastic strain of the surface layer in the sheared portion is calculatedbased on a Mises yield function represented by the following Formula(1).

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A sixth aspect of the invention is to provide the surface layer grainrefining hot-shearing method according to the first or second aspect ofthe invention, wherein the shearing of the steel sheet starts withinthree seconds after the steel sheet comes in contact with the die.

A seventh aspect of the invention is to provide the surface layer grainrefining hot-shearing method according to the first or second aspect ofthe invention, wherein the rapid cooling is performed when the steelsheet comes in contact with the die.

An eighth aspect of the invention is to provide the surface layer grainrefining hot-shearing method according to the first or second aspect ofthe invention, wherein the rapid cooling is performed when water jettingfrom a puncture formed in a contacting portion of the steel sheet withthe die passes through a groove provided in the contacting portion ofthe steel sheet.

A ninth aspect of the invention is to provide the surface layer grainrefining hot-shearing method according to the first or second aspect ofthe invention, wherein press forming not accompanying fracture of thesteel sheet is performed between the heating and the shearing of thesteel sheet.

A tenth aspect of the invention is to provide a workpiece obtained bysurface layer grain refining hot-shearing, including: a steel sheethaving a carbon content of 0.15% or more by mass, a surface layer of asheared portion of the steel sheet having a carbon content of 0.15% ormore by mass including a ferrite phase and a remainder, the surfacelayer being defined as a range up to 100 μm inside of the steel sheet ina normal direction of a shear plane from a fracture plane of the shearedportion; wherein the remainder includes at least one phase of a bainitephase, a martensite phase, or a residual austenite phase which have acrystal grain diameter of 3 μm or less, and includes cementite andinevitably generated inclusions; wherein the ferrite phase has anaverage grain size of 3 μm or less; wherein the surface layer contains5% or more grains by number having an aspect ratio of 3 or more; andwherein a region out of the range of 100 μm includes: martensite andinevitably generated inclusions; or bainite, martensite, and inevitablygenerated inclusions.

An eleventh aspect of the invention is to provide the workpiece obtainedby surface layer grain refining hot-shearing according to the tenthaspect of the invention, wherein, in the surface layer, the cementitehas a number density of 0.8 pieces/μm³ or less and the cementite has amaximum length of 3 μm or less.

A twelfth aspect of the invention is to provide the workpiece obtainedby surface layer grain refining hot-shearing according to the tenth oreleventh aspect of the invention, wherein a total area ratio of thebainite phase, the martensite phase, and the residual austenite phase,which are measured by an electron-beam backscattering diffraction (EBSD)method, is from 10% to 50% in the surface layer.

A thirteenth aspect of the invention is to provide a workpiece obtainedby surface layer grain refining hot-shearing, the workpiece produced by:heating and keeping a steel sheet having a carbon content of 0.15% ormore by mass in a temperature range of from Ac3 to 1400° C. toaustenitize the steel sheet; subsequently shearing the steel sheet in astate in which the steel sheet is placed on a die; and quenching byrapidly cooling the sheared steel sheet, wherein a start temperature ofthe shearing is set to be a temperature (° C.) obtained by adding atemperature of from 30° C. to 140° C. to a previously measured Ar3 ofthe steel sheet.

A fourteenth aspect of the invention is to provide a workpiece obtainedby surface layer grain refining hot-shearing, the workpiece produced by:heating and keeping a steel sheet having a carbon content of 0.15% ormore by mass in a temperature range of from Ac3 to 1400° C. toaustenitize the steel sheet; subsequently shearing the steel sheet in astate wherein the steel sheet is placed on a die; and quenching byrapidly cooling the sheared steel sheet, wherein a start temperature ofthe shearing is set to be a temperature (° C.) obtained by adding avalue, which is calculated by multiplying an amount of equivalentplastic strain of a surface layer in a sheared portion by a coefficientfrom 40 to 60, to a previously measured Ar3 of the steel sheet.

Advantageous Effects of Invention

According to a surface layer grain refining hot-shearing method and aworkpiece obtained by surface layer grain refining hot-shearing of theinvention, it is possible to suppress delayed fracture in a shearedportion and to provide a workpiece having excellent dimension accuracywithout increasing the shearing time and new steps.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a schematic diagram illustrating an example ofpunching-shearing by a punch and a die.

FIG. 1B is a schematic diagram illustrating an example oftrimming-shearing by a punch and a die.

FIG. 2 is a diagram illustrating an example of a sheared portion of asteel sheet.

FIG. 3 is a diagram illustrating a relation between a temperaturehistory and an Ar3 point.

FIG. 4A is a diagram illustrating a state of a hot-shearing apparatusused in Test A before shearing.

FIG. 4B is a diagram illustrating a state of the hot-shearing apparatusused in Test A during shearing.

FIG. 4C is a diagram illustrating a state of the hot-shearing apparatusused in Test A after shearing.

FIG. 5 is a diagram illustrating inclusions (a transmission electronmicroscope image observed by a replica method), which are observed by areplica method using a transmission electron microscope in ComparativeExample, in a surface layer of a sheared portion.

FIG. 6A is a diagram illustrating a region in which equivalent plasticstrain is averaged.

FIG. 6B is a diagram illustrating a region in which a fine structure inan actually hot-sheared portion is formed.

FIG. 7 is an example of metal structure (EBSD image) obtained by Example1.

FIG. 8 is an example of inclusions (a transmission electron microscopeimage observed by a replica method) of a metal structure obtained byExample 1.

FIG. 9A is a diagram illustrating a bending state of a hot-shearingapparatus used in Test B.

FIG. 9B is a diagram illustrating a shearing state of a hot-shearingapparatus used in Test B.

DESCRIPTION OF EMBODIMENTS First Embodiment

A surface layer grain refining hot-shearing method and a workpieceobtained by surface layer grain refining hot-shearing according to afirst embodiment of the invention will be described below.

First, general shearing will be described and a sheared portion of thesheared workpiece which is subjected to the shearing will be thendescribed.

As illustrated in FIGS. 1A and 1B, punching-shearing ortrimming-shearing is performed on a steel sheet 1 placed on a die 3 bylowering of a punch 2. At this time, as illustrated in FIG. 2, a shearedportion 8 of the steel sheet 1 is configured by (a) a shear drop 4 thatis formed in such a manner that the steel sheet 1 is totally pressed bythe punch 2, (b) a shear plane 5 that is formed in such a manner thatthe steel sheet 1 is drawn into a clearance between the punch 2 and thedie 3 (a gap between the punch 2 and the die 3) and is then locallystretched, (c) a fracture plane 6 that is formed in such a manner thatthe steel sheet 1 drawn into the clearance between the punch 2 and thedie 3 is fractured, and (d) a burr 7 that is generated on the backsurface of the steel sheet 1.

In the following description of the embodiment, the same components arealso denoted by the same reference numerals and the detailed descriptionthereof will be not presented.

In this embodiment, a term of “surface layer of the sheared portion” isused, and this refers to a region from the surface of the shearedportion up to 100 μm in a normal direction of the shear plane.

Hereinafter, first, the findings of the inventors on the hot shearingare described, the surface layer grain refining hot-shearing methodfound based on the findings is then described, and the workpieceobtained by surface layer grain refining hot-shearing formed by such ashearing method is finally described together with the operation of theshearing method.

In the hot shearing according to this embodiment, a steel sheet ofhigh-carbon region of 0.15% or more by mass is used. A transformationstart temperature (Ae3 point) in a state diagram from austenite toferrite of the steel sheet is from 800° C. to 900° C. A portion, whichis subjected to large plastic deformation in the austenite state, istransformed to ferrite without an occurrence of martensitetransformation even when being rapidly cooled. Therefore, when beingrapidly cooled after being sheared at a temperature range of anaustenite single phase based on the state diagram, almost entirely ofthe surface layer of the sheared portion having large plasticdeformation is transformed into ferrite and other portions, which arenot plastically deformed, are transformed into martensite. However, whenthe shearing temperature is high, dimension accuracy becomes poor due tothermal strain. In addition, there was a problem that variation inoccurrence of delayed fracture results from the plastically-deformedferrite at the time of the shearing at a temperature range in which theaustenite and the ferrite are mixed based on the state diagram.

Then, the inventors have experimented to perform the shearing on thesteel sheet which is subjected to a soaking treatment followed bychanging a temperature for starting the shearing (shearing starttemperature). With respect to the shearing start temperature, athermocouple was embedded at the center in a thickness direction of thesheet at a position spaced apart by 3 to 5 mm from a shearing positionof the steel sheet to measure the temperature at the start of shearing.Since the steel sheet is heat-released and thus lowered in thetemperature when coming in contact with a die, the shearing of the steelsheet started within three seconds after the steel sheet comes incontact with the die.

In this embodiment, the “die” refers to the die 3 and a pad 9 (see FIG.4A) to be used during the shearing. Furthermore, the meaning of “afterthe steel sheet comes in contact with the die” refers to the time afterthe steel sheet 1comes in contact with either of the die 3 or the pad 9.

As a result, the inventors found that there is a temperature range inwhich the delayed fracture does not occur on the sheared portion(fracture plane) of the steel sheet and the dimension accuracy isimproved and that this temperature range varies depending on shearingconditions or components of the steel sheet. The inventors also foundthat cooling control of the steel sheet before the shearing also affectsthe delayed fracture of the sheared portion (fracture plane) or thedimension accuracy of the workpiece.

The inventors found that fine bainite or fine martensite and fineresidual austenite are added in addition to fine ferrite and thatcementite reduces when the shearing start temperature is set to be anappropriate temperature as will be described below.

In general, the fine ferrite structure has toughness higher than themartensite structure. Therefore, when the fine ferrite structure havinghigh toughness is present in the surface layer of the sheared portion,the delayed fracture is suppressed.

The shearing start temperature having an appropriate temperature rangewas obtained by considering temperature changes in the hot shearing andfurther calculating the size of shearing strain.

The steel sheet was first heated to 950° C. and after keeping it for 90seconds and then cooling it in a state being placed on four pointedneedles (hereinafter, sometimes referred to as a “pin support”), thetransformation temperature of the steel sheet was measured. Thetemperature was measured by the thermocouple embedded in the steelsheet.

The measured Ar3 point is a temperature that starts to transform to aBBC crystalline structure such as ferrite from the austenite structureof an FCC crystal at a finite cooling rate rather than the assumptionthat the cooling rate is zero as in the state diagram.

The measured Ar3 point was significantly different in the range of from200 to 300° C. from a transformation temperature (Ae3 point) at whichaustenite was changed to ferrite as illustrated in the state diagram.Further, the Ar3 point measured in a surface contact state with the die(quenching is inadequate, but the cooling rate is faster compared to thecase of the pin support) was as low as about 400° C. compared to the Ae3point, that is, was as low as about 100° C. compared to the case of thepin support.

The fact that the Ar3 point is lower than the Ae3 point is commontechnical knowledge in the field of metallic materials. However, aquantitative difference between the Ar3 point and the Ae3 point is notclear. By testing of the inventors, it was clear that the significantdifference between the Ar3 point and the Ae3 point is present in the hotshearing as described above.

For reference, results of measurement of the Ar3 point by the abovemeasuring method (pin support) are illustrated in FIG. 3. The steelsheet to be mainly used had a sheet thickness of 1.5 mm. The range ofthe thickness of the steel sheet to be used in the shearing is of aboutfrom 0.5 mm to 3.0 mm. Since the Ar3 point is the transformation starttemperature at which the austenite is changed to the ferrite, it is notnecessary to include shearing and a quenching (rapid cooling) process onthe measurement of the Ar3 point. Accordingly, the quenching process isnot included in the graph of FIG. 3.

In FIG. 3, initially, the cooling rate was 7° C./s, and the cooling ratehas sharply declined when the time has elapsed for 50 seconds from acooling start. A temperature (about 680° C.) of the steel sheet at whichthe cooling rate of the steel sheet is equal to or less than 1° C./s isidentified as the transformation temperature (Ar3 point). At the time ofthe measurement of the Ar3 point, the steel sheet is cooled to roomtemperature as it is, but, in actual fact, the shearing starts at atemperature higher than the Ar3 point and the quenching process is thenperformed.

In this embodiment, an Ar3 temperature measured using the same method asin the case of the above pin support under placing conditions of a sheetto be actually sheared is defined as the “measured Ar3 (of the steelsheet)”. The cooling rate is generally about from 5° C./s to 30° C./s(state of cooling by leaving) at the time of the measurement in manycases.

As long as appropriate hot-shearing conditions are ascertained byperforming the above experiment as a preliminary test, when performingappropriate soaking temperature management of the steel sheet and timemanagement up to the shearing start after placing the steel sheet in thedie at steps of an actual mass production process, it is not necessaryto perform the operation after preparing the die in which thethermocouple is embedded and measuring a surface temperature of thesteel sheet to be sheared at the time of the shearing start for everyshearing. In the case of performing the operation by measuring thesurface temperature of the steel sheet in the mass production process,the surface temperature of the steel sheet may be measured immediatelybefore the hot-shearing using a radiation thermometer.

From the fact that the plastic deformation caused by the shearing isrelated to the structure of the sheared portion as described above, theinventors derived plastic strain in the vicinity of the sheared portionby numerical calculation. Here, the plastic strain was evaluated asequivalent plastic strain.

From the fact that the actual shearing is performed at a range higherthan the measured Ar3 temperature, as a premise of the calculation, thenumerical value of mechanical characteristics such as deformationresistance of the steel sheet was defined as a value of austenite. Inaddition, the temperature dependence of the mechanical characteristicsof austenite was obtained using an actual measurement value in a hottensile test (after heating the steel sheet to a temperature higher thanor equal to the Ac3 point, the steel sheet is cooled by leaving to apredetermined temperature, and then a tensile test is performed) of22MnB5 equivalent steel which is widely used for hot stamping. Such atemperature dependence is disclosed in, for example, “Hongsheng Liu, JunBao, Zhongwen Xing, Dejin Zhang, Baoyu Song, and Chengxi Lei; “Modelingand FE Simulation of Quenchable High Strength Steels Sheet Metal HotForming Process”, Journal of Materials Engineering and Performance, Vol.20(6), 2011, pp. 894 to pp. 902” (hereinafter, sometimes referred to as“Non-Patent Literature 3”), and practitioners may use values disclosedin this Literature without actually measuring the values.

The plastic strain obtained by the numerical calculation is largest atthe surface of the shearing surface, and becomes smaller moving awayfrom the surface. Furthermore, it was found that an occurrence region ofthe equivalent plastic strain of 100% or more at the sheared portioncoincides with an actual occurrence region of the fine structure in apredetermined temperature range.

With respect to the values obtained by the numerical calculation, it isconcerned that variation is caused by analysts. Therefore, the inventorsperformed the numerical calculation using steel grades, analyst, andsoftware in plural ways. As a result of the numerical calculation, theinventors obtained the result that the temperature range at which theoccurrence region (distance) of the equivalent plastic strain of 100% ormore in the normal direction of the shear plane at the sheared portioncoincides with the occurrence region of the fine structure in the normaldirection of the shear plane is a temperature range higher byapproximately 30 to 140° C. than the measured Ar3.

Here, at a temperature range higher than a temperature obtained byadding 140° C. to the measured Ar3 (hereinafter, sometimes referred toas “higher than Ar3+140° C.”), the occurrence region of the equivalentplastic strain of about 100% in the normal direction of the shear planeon the sheared portion which is obtained by calculation becomes largerthan the actual fine region on the sheared portion of the workpiece. Asa result of analysis of the fine structure region, the region was mainlyconfigured by ferrite and carbide. On the other hand, other regionsexcept the surface layer are configured by a martensite structure.

The ferrite and the martensite have a different volume, respectively,from the difference of a crystal structure and a solid-solution state ofelement. Therefore, when the fine structure region is widely formed onthe surface layer of the sheared portion and most of the fine structureis configured by ferrite, the boundary area between the fine ferrite andthe fine martensite increases. As a result, the dimension accuracy ofthe workpiece deteriorates. In consideration of the thermal strain, thedimension accuracy of the workpiece deteriorates as the shearing starttemperature becomes higher.

Furthermore, when the shearing start temperature is lower than atemperature obtained by adding 30° C. to the measured Ar3 (hereinafter,sometimes referred to as “lower than Ar3+30° C.”), the actual fineregion is smaller than the occurrence region of the equivalent plasticstrain of 100% or more. Since the occurrence region of the equivalentplastic strain of 100% or more becomes smaller, the actual finestructure region smaller than such a region becomes further smaller. Atthe temperature lower than “Ar3+30° C.” which is measured, a part ofaustenite starts to transform into ferrite by the influence of internalheat distribution, and such ferrite is plastically deformed by theshearing. Consequently, the inventors found that residual stress isexcessively large on the surface of the sheared portion of the workpieceand thus the risk of the delayed fracture increases.

On the other hand, when the shearing start temperature is higher than“Ar3+30° C.”, the steel sheet is subjected to the shearing beforeaustenite starts to transform into ferrite, so excessive residual stresson the sheared portion due to ferrite is avoided.

Based on the above findings, the surface layer grain refininghot-shearing method according to this embodiment was configured asfollows.

First, a shearing machine used in the test will be briefly described. Asillustrated in FIG. 4A, a shearing machine 10 includes the die 3 onwhich the steel sheet 1 is placed, a pad 12 that is disposed on the die3 to press the steel sheet 1 placed on the die 3, and a punch 2 that isdisposed inside the pad 12 and is inserted into a puncture 14 of the die3 to punch a predetermined range of the steel sheet 1.

First, the steel sheet 1 having the carbon content of 0.15% or more bymass is placed on the die 3 after being heated to the range of from Ac3to 1400° C. higher than the shearing start temperature in the range offrom Ar3+30° C. to Ar3+140° C. and being subjected to a soakingtreatment (see FIG. 4A).

Then, as illustrated in FIG. 4B, after the steel sheet 1 on the die 3 ispressed by the pad 12, the steel sheet 1 is subjected to the shearing bythe punch 2. After the steel sheet 1 is placed on the die 3, theshearing of the steel sheet 1 starts within three seconds. By control ofthe time (shearing start time) until the shearing starts after the steelsheet 1 is placed on the die 3, the temperature of the steel sheet 1during the shearing is controlled in the range of from Ar3+30° C. toAr3+140° C.

As illustrated in FIG. 4C, a predetermined range of the steel sheet 1 ispunched by the punch 2, the punched steel sheet 1 is rapidly cooled andquenched by the die 3 and the pad 12, and thus a shearing-workpiece isformed.

Operation of the surface layer grain refining hot-shearing methodaccording to this embodiment as described above and the workpieceobtained by surface layer grain refining hot-shearing (hereinafter,sometimes referred to as a “workpiece”) formed by this shearing methodwill be described.

In the sheared portion 8 of the workpiece (steel sheet) formed in thismanner, the surface layer of the sheared portion 8 defined as the rangeup to 100 μm inside of the steel sheet in a normal direction of theshear plane 5 includes a ferrite phase forming at least a portion of thefracture plane and the remainder, and the remainder has a bainite phase,a martensite phase, a residual austenite phase, and cementite andinevitably generated inclusions. The ferrite phase, the bainite phase,the martensite phase, and the residual austenite phase which are formedin the surface layer of the sheared portion 8 have an average grain sizeof 3 μm or less, respectively. The surface layer of the sheared portion8 contains 5% or more grains by number having an aspect ratio of 3 ormore. In addition, other regions except the surface layer of the shearedportion 8 includes a mixed structure of an inevitably generatedinclusion and martensite or a mixed structure of martensite, bainite,and an inevitably generated inclusion.

That is, since the workpiece is formed by the shearing of the steelsheet 1 heated to the temperature of from Ar3 point+30° C. to Ar3point+140° C., a fine ferrite structure, a fine martensite structure, afine bainite structure, and a fine residual austenite structure areformed in the surface layer of the sheared portion 8 (fracture plane 6)(see FIG. 2). FIG. 6B illustrates the steel sheet 1 which has actuallybeen subjected to the shearing. As illustrated in FIG. 6B, a finestructure 11 is formed from the fracture plane 6 toward the shear plane5 in the sheared portion 8 in the surface layer, but the fine structureis formed particularly up to a depth of about 100 μm from the surface inthe fracture plane 6.

The fine ferrite structure has generally higher toughness than themartensite structure. Accordingly, since the fine ferrite structure ofthe high toughness is present in the surface layer of the shearedportion 8 (fracture plane 6), occurrence of the delayed fracture in thesheared portion 8 (fracture plane 6) due to the delayed fracture issuppressed.

As will be described below, in the workpiece according to thisembodiment, the occurrence of the delayed fracture in the shearedportion 8 (fracture plane 6) can be suppressed by the fine martensitestructure, the fine bainite structure, and the fine residual austenitestructure which are formed in the surface layer of the sheared portion 8(fracture plane 6).

For reference, FIG. 7 illustrates a structure photograph of the surfacelayer of the sheared portion obtained by an EBSD of this embodiment.

In FIG. 7, a black part indicates a bainite phase, a martensite phase,or a residual austenite phase. As in the photograph, although crystalgrains having the aspect ratio of 3 or more are present, the delayedfracture does not occur for reasons which will be described below.

The “grain size” used herein means a circle diameter, that is, a circleconversion diameter (circle equivalent diameter) when an area of eachferrite crystal grain, which is observed in a cross section along thethickness direction of the steel sheet in the normal direction of theshear plane, is replaced by a circle of the same area.

The bainite phase, the martensite phase, or the residual austenite phaserather than the single phase of the fine ferrite phase is present in thesurface layer of the sheared portion 8. Generally, the bainite phase,the martensite phase, or the residual austenite phase present in theferrite phase traps diffusible hydrogen that causes the delayedfracture. Therefore, when these phases are present in the fine ferritephase, it is possible to obtain an effect of suppressing the delayedfracture.

In addition, when the bainite phase, the martensite phase, or theresidual austenite phase becomes finer to be 3 μm or less, sites fortrapping the diffusible hydrogen further increase, and thus the delayedfracture is further suppressed.

On the other hand, the cementite has a small effect of trapping thediffusible hydrogen and can be a start point of the occurrence of thedelayed fracture, so it is preferable that the cementite becomessmaller.

In order for the remainder to have the fine bainite phase, martensitephase, and/or residual austenite phase having the grain size of 3 μm orless, ferrite having an aspect ratio of more than 3 inevitably appeared.As a result of analysis using a transmission electron microscope, theferrite having the aspect ratio of more than 3 is in a state whereplastic deformation little occurs or is small, but is not in a state ofbeing plastically deformed and stretched as described in PatentLiterature 6, so the ferrite did not adversely affect resistance to thedelayed fracture. While the details of the operation is not clear, inorder for the remainder to have the bainite phase, the martensite phase,or the residual austenite phase described above, the ferrite structurehaving the aspect ratio of more than 3 is essentially present.

In order to also make these structures, it is necessary to adjust theshearing temperature to a temperature range of from Ar3+30° C. toAr3+140° C. It is considered that since the steel sheet is cooled at acertain cooling rate, the austenite structure remains at the shearingtemperature, but the appropriate amount of shearing strain is added andtransformation nuclei to transform into other phases other than themartensite is already generated. In this case, the cooling ratecontributes to any phase transformation.

The cooling rate is fast when the temperature exceeds Ar3+140° C., andthe austenite becomes a supercooled state during cooling (temperature islower than a temperature range at which structure morphology can bepresent) when the shearing strain is applied to the extent in whichtransformation to martensite cannot occur. In such a case, austenite iseasily transformed into a fine ferrite structure.

On the other hand, when the temperature is equal to or lower thanAr3+140° C., grains are formed in which transformation to ferrite doesnot occur and transformation to martensite also does not occur under theinfluence of shearing strain. Such grains become a bainite phase. Inaddition, grains are also present in which shearing strain is small andtransformation to martensite occurs. Additionally, the transformation tothe non-uniform three phases partially induces enrichment of carbon toaustenite, and such austenite becomes residual austenite in order to bestable even at room temperature. Since these phases occur between thefine ferrite grains, the phases themselves also become finer to be 3 μmor less.

In order to stably form these structures, the shearing of the steelsheet preferably starts within three seconds after the steel sheet comesin contact with the die. When the shearing starts after three seconds,scale occurs on the surface of the steel sheet and the contact of thedie with the steel sheet becomes non-uniform. When heat irregularityoccurs due to the non-uniform contact, variation in cooling condition ofthe sheared portion is caused.

In addition, FIG. 5 illustrates cementite distribution in the surfacelayer of the fracture plane when the steel sheet disclosed in PatentLiterature 6 is subjected to the shearing at a temperature higher thanAr3 point+140° C. In Patent Literature 6, since the shearing starttemperature is simply set to only a temperature range of from 400° C. to900° C., the shearing start temperature also includes the case of beinghigher than Ar3+140° C. In this case, for example, as illustrated inFIG. 5, cementite C (black parts excluding circles) has a number densityof 0.8 pieces/μm³ or more and the maximum length of 3 μm or more.

On the other hand, in the case of this embodiment, cementite (blackparts excluding circles) in the surface layer of the fracture plane ofthe steel sheet has a number density of 0.8 pieces/μm³ or less and themaximum length of 3 μm or less as indicated in test results (FIG. 8) tobe described below. According to the experience of the inventors, whenthe number of cementite is small to this extent and the size ofcementite is also small, the cementite itself does not almost cause aproblem of being a start point of the occurrence of the delayedfracture.

As illustrated in FIG. 7, a total area ratio of the bainite phase, themartensite phase, or the residual austenite phase, which is measured byobservation in the range up to 100 μm inside of the steel sheet in thenormal direction of the shear plane from the fracture plane in thesheared portion of the steel sheet using an electron-beam backscatteringdiffraction (EBSD) method, is from 10% to 50%.

As for this, according to the experience of the inventors, when thetotal area ratio of these phases is less than 10%, it is not possible tosufficiently perform the storage of the diffusible hydrogen and the riskof the delayed fracture increases. On the other hand, when the totalarea ratio of these phases exceeds 50%, the ratio of the fine ferrite inthe surface layer of the fracture plane reduces, whereby the effect oftoughness improvement due to the fine ferrite decreases and the risk ofthe delayed fracture increases. Although the effect of the inventiondoes not immediately disappear when the total area ratio of these phasesis out of such a range, the total area ratio of these phases is morepreferably within such a range.

A method of rapidly cooling the steel sheet 1 after the shearing is notlimited to rapid cooling by the contact of the die (die 3 and pad 12)with the steel sheet 1 as in this embodiment and, for example, the steelsheet 1 may be rapidly cooled by allowing the steel sheet 1 to come indirectly contact with water. Examples of the method of allowing thesteel sheet 1 to come in contact with water may include a method ofpassing cooling water through a groove formed in a contacting portion ofthe steel sheet with the die.

Even in the case of performing the shearing after press forming, as inthe workpiece of this embodiment, it is possible to suppress the delayedfracture of the sheared portion to form a workpiece with dimensionaccuracy.

Second Embodiment

A surface layer grain refining hot-shearing method according to a secondembodiment of the invention will be described. The same components as inthe first embodiment are denoted by the same reference numerals, and thedetailed description thereof will not be presented. In addition, aworkpiece obtained by surface layer grain refining shearing formed bythe surface layer grain refining hot-shearing method according to thisembodiment is the same as in the first embodiment, so operationaleffects thereof will not be described.

The inventors found that the temperature range at which the occurrenceregion of about 100% equivalent plastic strain in the normal directionof the shear plane in the sheared portion coincides with the occurrenceregion (distance) of the fine ferrite structure, the fine martensitestructure, the fine bainite structure, or the fine residual austenitestructure in the normal direction of the shear plane is obtained when atemperature (° C.) obtained by adding a value, which is calculated bymultiplying the amount of equivalent plastic strain of the surface layerin the sheared portion by a coefficient from 40 to 60, to the measuredAr3 is set as a shearing start temperature.

In this embodiment, it was considered that the following value wasappropriate to use as the amount of equivalent plastic strain of thesurface layer in the sheared portion.

As illustrated in FIG. 6A, an average value-of the amounts of plasticstrain obtained by calculation at a region A (within a thick line frame)in the range of from 5 to 20% of a thickness H of the steel sheet 1 fromthe shear plane 5 of the sheared portion 8 to the inside of the steelsheet 1 in the normal direction of the shear plane 5 and in the range offrom 20% to 50% of the thickness H of the steel sheet 1 in the thicknessdirection of the steel sheet 1 from a bottom 12 on the burr 7 side ofthe sheared portion 8 was used as the amount of equivalent plasticstrain of the surface layer in the sheared portion.

By setting the region A in this way, the inventors found that the amountof equivalent plastic strain having a small influence by differences inanalyst or analysis condition was obtained. This value is considered tobe a reasonable numerical value as the amount of equivalent plasticstrain as will be described below, but other values of correction strainmay be used according to a calculation unit.

The amount of equivalent plastic strain of the surface layer in thesheared portion used a value obtained by the calculation at atemperature range of from 500° C. to 800° C. It was confirmed that theamount of equivalent plastic strain of the surface layer becomesapproximately constant at this range.

the reason that a lower limit of 40 is set for the coefficient to bemultiplied by the amount of equivalent plastic strain is due toconsideration of differences in the coefficient due to a steel grade anderrors in numerical calculation. By repetitive experiment and numericalcalculation, the fine ferrite structure, the fine martensite structure,the fine bainite structure, or the fine residual austenite structureappeared even in the case of being out of this coefficient range, butthe inventors obtained 40 as the lower limit of the coefficient in whichappearance probability becomes higher.

In addition, the reason why the upper limit of the coefficient to bemultiplied by the amount of equivalent plastic strain is set to 60 isthat the dimension accuracy of the workpiece deteriorates when theshearing temperature is too high. This reason is considered that theregion of the fine structure in the surface layer becomes wider as thetemperature becomes higher, but the dimension accuracy deterioratesafter cooling because a difference in density between the surface layerand another region adjacent to the surface layer is large and thethermal strain also increases.

In a case in which the difference between a workpiece dimension and adesign dimension of the workpiece generally falls within the range of−0% +5% of the design dimension, the defective rate of product islowered to the extent of being economically acceptable and thus problemssubstantially disappear. Thus, as a result of trial and error, such anupper limit was determined.

The measured Ar3 point of the steel sheet should be previously measuredby a temperature drop history at the thermocouple or the like in a statein which the steel sheet is placed on the die to be actually used. Thethermocouple is embedded in the die, and it is preferable to cause athermocouple sensor to come in directly contact with the steel sheetwhich is a member to be sheared. This reason is that the measured Ar3point varies depending on the cooling rate of the steel sheet. Asillustrated in FIG. 3, it is widely known that the measured Ar3 point ismeasured as a point at which a temperature lowering rate varies. Thistechnique is also used in Tests A and B to be described below.

In this embodiment, it is important to calculate the equivalent plasticstrain of the sheared portion. In the hot-shearing, the metal-structuretransformation inevitably occurs during or immediately after theshearing, and thus it is not possible to measure the equivalent plasticstrain. Therefore, a shearing simulation is performed by analysis usinga finite element method (FEM), and thus the equivalent plastic strain iscalculated.

In the shearing simulation, the plastic strain is steeply changed. Forthis reason, calculation results of the plastic strain of the surfacelayer in the sheared portion are likely to differ depending on analystsor analysis conditions. In order to reduce the influence of theseanalysts or analysis conditions, it is preferable to set a constant FEManalysis region and to average and calculate the equivalent plasticstrain within the region.

The inventors have set the region as a result of trial and error. FIG.6A illustrates the region in which the equivalent plastic strain isaveraged. As illustrated in FIG. 6A, the region A (within the thick lineframe), in which the equivalent plastic strain is averaged, was set inthe range of from 5 to 20% of the thickness H (see FIG. 4) of the steelsheet 1 from the shear plane 5 of the sheared portion 8 to the inside ofthe steel sheet 1 in the normal direction of the shear plane 5 and inthe range of from 20% to 50% of the thickness H of the steel sheet 1 inthe thickness direction of the steel sheet 1 from the bottom 12 on theburr 7 side of the sheared portion.

During the simulation, since the temperature change sequentially occurs,it is necessary to perform repetitive calculation in such a manner that:a tentative shearing start temperature is set; the equivalent plasticstrain is calculated based on the tentative shearing start temperature;and a true shearing start temperature is determined based on thecalculated equivalent plastic strain. Such calculation requires costs.

As a result of the calculation with several levels by the inventors, itwas found that approximation can be performed when a numericalsimulation is once performed based on stress-strain diagram at any ofthe steel sheet temperature of from 500° C. to 800° C.

As a premise of the calculation, when the shearing is performed at therange higher than the measured Ar3 temperature, numerical values ofmechanical characteristics such as rigidity of the steel sheet at thattime were defined as values of austenite.

During the simulation, the shearing start temperature can be calculatedwithout any problem when the equivalent plastic strain is calculated bya Mises yield function on the supposition of an isotropy withoutconsidering an anisotropy in particular.

An increment in equivalent plastic strain “dε-P” by the Mises yieldfunction is represented by the following formula when a materialcoordinate system is defined as x, y, and z, and the equivalent plasticstrain is given as an integral of this increment.

$\begin{matrix}{{d\; {\overset{\_}{ɛ}}_{P}} = \sqrt{\frac{2}{3}\left( {{d\; ɛ_{xx}^{2}} + {d\; ɛ_{yy}^{2}} + {d\; ɛ_{zz}^{2}} + {2d\; ɛ_{xy}^{2}} + {2d\; ɛ_{yz}^{2}} + {2d\; ɛ_{zx}^{2}}} \right)}} & (1)\end{matrix}$

As described above, in the shearing method according to this embodiment,the structures such as the fine ferrite are formed in the surface layerin the sheared portion and the occurrence of the delayed fracture in thesheared portion (fracture plane) is suppressed when the steel sheet issubjected to the shearing at the calculated shearing start temperature,and it is possible to suppress the thermal strain or the like and ensurethe dimension accuracy of the workpiece by allowing the shearing starttemperature to be within the predetermined range.

In particular, since the predetermined range region A is set tocalculate the amount of equivalent plastic strain in the shearedportion, it is possible to calculate the amount of equivalent plasticstrain having a small error.

During the FEM simulation for calculating the equivalent plastic strain,since the temperature change sequentially occurs, it was necessary toperform repetitive calculation in such a manner that: the equivalentplastic strain was calculated based on the tentative shearing starttemperature; and the true shearing start temperature was determinedbased on the calculated equivalent plastic strain. In this embodiment,however, since the approximation can be performed when a numericalsimulation is only once performed based on stress-strain diagram at anyof the steel sheet temperature of from 500° C. to 800° C., thecalculation is simplified.

Since the equivalent plastic strain is calculated by the Mises yieldfunction on the supposition of an isotropy, the calculation is furthersimplified.

The method of calculating the amount of equivalent plastic straindisclosed in the surface layer grain refining hot-shearing methodaccording to the second embodiment is applicable to the calculation ofthe amount of equivalent plastic strain in the surface layer grainrefining hot-shearing method according to the first embodiment.

EXAMPLES

Next, Examples of the invention will be described. However, shearingconditions in Examples are examples adopted to confirm feasibility andan effect of the invention and the invention is not limited to theseshearing conditions. The invention can adopt various shearing conditionsas long as the object of the invention is achieved within a range of notdeparting from the gist of the invention.

(Test A)

Using the shearing machine 10 illustrated in FIGS. 4A to 4C, after thehigh-strength steel sheet 1 (200 mm×150 mm) of steel grades A to Chaving compositions indicated in Table 1 is placed on the die 3, thepunch 2 together with the pad 12 approach the top of the steel sheet 1from the above. The steel sheet 1 is pressed by the pad 12 and the steelsheet 1 is subjected to the shearing by the punch 2 (width of 65 mm) atthe same time. The sheared steel sheet 1 is rapidly cooled by the die(die 3 and pad 12). Shearing conditions are as indicated in Table 2. Aclearance between the punch 2 and the die 3 was set to be 0.15 mm.

Except for Comparative Examples, the keeping time until the shearing ofthe steel sheet 1 starts after coming in contact with the die 3 was setto be from 0.5 seconds to 3 seconds. The shearing start temperatures inTable 2 are temperatures obtained within the range of the keeping time.

The thickness of the steel sheets used in Examples was set to be 1.5 mm.The thickness of the steel sheet applicable to the invention has therange of from about 0.5 mm to 3 mm.

The measured Ar3 point of each steel sheet was obtained by themeasurement of the temperature history at the time when the steel sheetheated to 950° C. is cooled in contact with the top of the die on theshearing machine (a temperature at which the cooling rate of the steelsheet was 1° C./sec. or less before the temperature of the steel sheetwas lowered to the room temperature was regarded as the Ar3 point).

For estimation of the equivalent plastic strain, shearing simulation, inwhich deformation resistance was input when the steel sheet is 750° C.,was performed by a finite element method using Abaqus/Standard made byDassault Systemes Co, which is commercial software. In this case, theMises yield function was used, and the analysis region in the vicinityof a tool cutting edge was defined as a quadrilateral completeintegration element of 0.02 mm×0.04 mm. In addition, remeshing wasperformed every 0.05 mm punching press. The fracture was defined by aductile fracture model of Hancock & Mackenzie, and the rigidity ofelements satisfying conditions was zero. Parameters of the ductilefracture model were fitted based on a shear plane ratio which wasactually observed in certain conditions. The equivalent plastic strainwas used which was averaged in the region A set in the range of 10% ofthe thickness H of the steel sheet 1 from the shear plane 5 of thesheared portion 8 in the normal direction of the shear plane 5 and inthe range of 30% of the thickness H of the steel sheet 1 in thethickness direction of the steel sheet 1 from the bottom 12 on the burr7 side of the sheared portion 8 (see FIG. 6A).

A length of a scrap 16 (see FIG. 4C) punched out after the shearing wasevaluated as the dimension accuracy. Unless a dimensional error occurs,the length of the scrap 16 after the shearing should be 65 mm. Thus,values are obtained in such a manner that the error in length of thescrap 16 after the shearing is divided by 65 and is then converted intopercentage (×100) are disclosed as the dimensional error in Table 2.

TABLE 1 (% by mass) Steel grade C Si Mn B Cr A 0.22 0.22 1.20 0.002 0.16B 0.16 0.40 1.00 0.001 0.23 C 0.25 0.21 1.24 0.002 0.34

TABLE 2 Steel sheet Steel-sheet heating Amount of Presence or Steel TimeRapidly equivalent Shearing start absence of Dimensional grade Ar3(° C.)Temperature(° C.) (min.) cooling plastic strain Coefficienttemperature(° C.) cracks error (%) Example 1 A 580 950 1.5 Water 2.0 50680 Absence 2.0 Example 2 A 580 950 1.5 Die 2.0 60 700 Absence 4.2Example 3 A 580 1000 1.0 Die 2.0 50 680 Absence 1.1 Example 4 B 620 9501.5 Water 2.5 50 745 Absence 3.0 Example 5 B 620 950 1.5 Water 2.5 40720 Absence 2.7 Example 6 C 570 950 1.5 Water 1.8 50 660 Absence 2.3Comparative A 580 950 1.5 Water 2.0 10 600 Presence 1.8 Example 1Comparative B 620 950 1.5 Water 2.5 −10 595 Presence 1.8 Example 2Comparative A 580 950 1.5 Water 2.0 85 750 Absence 5.1 Example 3Comparative B 620 950 1.5 Water 2.5 80 820 Absence 6.3 Example 4Comparative C 570 950 1.5 Water 1.5 100 720 Absence 5.1 Example 5

The test was performed three times for each Examples and ComparativeExamples. With respect to the presence or absence of the delayedfracture, it was evaluated that the delayed fracture was present whendelayed fracture occurs even once. In addition, the dimensional errorwas an average value of three measured values.

In Examples 1 to 6, it can be understood that the occurrence of thedelayed fracture in the sheared portion (fracture plane) is suppressedand the dimension accuracy of the workpiece is improved.

A microstructure in the range of 100 μm from the fracture plane of thesheared portion in Example 1 will be described with reference to FIG. 7(EBSD, microstructure image) and FIG. 8 (image of an extraction replicasample observed by the transmission electron microscope).

As illustrated in FIG. 7, it was confirmed that the microstructureincludes ferrite, bainite, martensite, residual austenite, cementite,and inclusions derived from alloy elements other than iron as a resultthe EBSD analysis, EDS (characteristic energy dispersion type X-rayanalysis), and electron diffraction analysis of the transmissionelectron microscope.

Specifically, FIG. 7 illustrates the microstructure image observed bythe EBSD in a state where a cross-section sample of Example 1 along thethickness direction of the steel sheet in the normal direction of theshear plane in the sheared portion is embedded in a hard resin and isthen subjected to polishing and electropolishing. In addition, FIG. 8illustrates the image observed by the transmission electron microscopeof the sample of Example 1 which is prepared by an extraction replicamethod using an SPEED method (Potentiostatic Etching by ElectrolyticDissolution: potentiostatic electrolysis method in nonaqueous solvent).

As illustrated in FIG. 7 (EBSD microstructure image), in the surfacelayer of the fracture plane in the range of 100 μm in the normaldirection of the shear plane from the fracture plane, the grain size offerrite (parts excluding black in FIG. 7) F was as very small as 3 μm orless and the grain size of BMA (black part in FIG. 7) includingmartensite, bainite, or residual austenite was also 3 μm or less. Thecrystal grain having the aspect ratio exceeding 3 was also seen in thisrange and the ratio was about 6% by number.

The same microstructure was observed in any of Examples 2 to 6. Duringthe identification of the microstructure, five points of field-of-viewof 8.0×20 μm were randomly photographed for each Example, in the rangeof 100 μm from the surface of the fracture plane.

Furthermore, as illustrated in FIG. 8, it can be seen that the ratio ofcementite (black parts excluding circles) C in Example 1 is very small.In Example 1, the number density of the cementite was 0.8 pieces/μm³,and the maximum length of the observed cementite was 3 μm or less. Inorder to determine a state of cementite distribution, five points offield-of-view of 9.5×7.5 μm from the surface layer of the shearedportion were randomly photographed for each condition. This was the samein any of Examples 2 to 6.

In Comparative Examples 1 to 5, on the other hand, a mixed structure(Comparative Examples 1 and 2) of bainite and martensite not includingferrite or a single phase of ferrite (Comparative Examples 3 to 5) wasobserved. In Comparative Examples 1 and 2, cementite and inclusion washardly observed in almost same manner as illustrated in FIG. 8. InComparative Examples 3 to 5, however, the cementite (see FIG. 5, blackpart excluding circles) C having very high number density greatlyexceeding 0.8 pieces/μm³ as illustrated in FIG. 5 was observed.

An experiment was performed in a state where other conditions except forthe shearing start temperature were the same as in Example 1, and thekeeping time until the shearing of the steel sheet starts after beingcooled in contact with the die 3 and the pad 9 (also referred to as adie) was set to be 3.5 seconds. In this case, the shearing starttemperature was also (Ar3+30° C.) or higher, the delayed fractureoccurred once in three repetitive experiments. As a result ofobservation the surface of the shearing surface of the resultingworkpiece, in the range of 100 μm from the shear plane, the structure ofthe surface layer of the sheared portion in the workpiece without anoccurrence of the delayed fracture was configured to include: ferrite ofwhich the grain size was as very small as 3 μm or less; and martensite,bainite, or residual austenite of which the grain size was also 3 μm orless. The crystal grain having the aspect ratio exceeding 3 was alsoseen and the ratio was about 7% by number.

In the range of 100 μm from the shear plane, however, the structure ofthe surface layer of the sheared portion in the workpiece withoccurrence of the delayed fracture was configured to include: ferrite ofwhich the grain size was about 5 μm; and martensite, bainite, orresidual austenite of which the grain size was also 5 μm. In the surfacelayer of the sheared portion, the crystal grain having the aspect ratioexceeding 3 was also seen and the ratio was about 7% by number.

(Test B)

A shearing machine 20 includes: a die 3 which is formed with a hole 22for bending and forming and a puncture 24 for punching deformation onthe bottom of the hole 22 and in which the steel sheet 1 is placed; apunch 2 which is inserted into the hole 22 to cause bending deformationof the steel sheet 1; and a movable die 26 which is incorporated intothe punch 2 and is inserted into the puncture 24 after the bendingdeformation to form a puncture (shearing) in a predetermined range ofthe steel sheet 1.

By simulating press forming not accompanying fracture of the steelsheet, the shearing machine 20 formed the heated steel sheet 1 in a hatshape by initially driving the punch 2 after the steel sheet 1 wasplaced on the die 3 (see FIG. 9A). Thereafter, a test of punching thesteel sheet 1 using a movable die 13 to have a diameter of 20 mm wasperformed (see FIG. 9B).

Except for Comparative Examples, the time until the shearing of thesteel sheet 1 starts after coming in contact with the movable die 26 wasfrom about 0.1 seconds to about 0.5 seconds.

A clearance between the punch 2 and the die 3 was set to be 0.15 mm andthe measured Ar3 was identified from a thermal history after the hatforming. The equivalent plastic strain was calculated in the same way asin Test A. Shearing conditions indicated in Table 3 were adopted.

An evaluation method in Test B is also the same as that in Test A.

By the way, the dimension accuracy in Test B was evaluated by a diameterof a punch hole after the shearing. When the dimensional error does notoccur, the diameter of the punch hole of the steel sheet 1 after theshearing should be 20 mm. Thus, values are obtained in such a mannerthat the error in diameter of the punch hole after the shearing isdivided by 20 and is then converted into percentage (×100) and thevalues are disclosed as the dimensional error in Table 3 which indicatesan implementation result of this test.

TABLE 3 Steel sheet Steel-sheet heating Amount of Presence or Steel TimeRapidly equivalent Shearing start absence of Dimensional grade Ar3(° C.)Temperature(° C.) (min.) cooling plastic strain Coefficienttemperature(° C.) cracks error (%) Example 7 A 420 950 1.5 Water 2.0 40500 Absence 1.1 Example 8 A 420 950 1.5 Die 2.0 60 560 Absence 1.1Example 9 B 480 950 1.5 Water 2.5 40 580 Absence 1.2 Example 10 C 460950 1.5 Water 1.8 40 532 Absence 1.1 Comparative A 420 950 1.5 Water 2.010 440 Presence 0.7 Example 6 Comparative B 480 950 1.5 Water 2.5 10 475Presence 0.8 Example 7 Comparative C 460 950 1.5 Water 1.8 10 478Presence 0.5 Example 8 Comparative A 420 950 1.5 Water 2.0 90 600Absence 2.3 Example 9 Comparative B 450 950 1.5 Water 2.5 80 650 Absence2.8 Example 10 Comparative C 460 950 1.5 Water 1.8 100 640 Absence 2.8Example 11

In Examples 7 to 10, it can be understood that the occurrence of thedelayed fracture in the sheared portion (fracture plane) is suppressed.

In Examples 7 to 10 indicated in Table 3, the microstructure in thesurface layer of the sheared portion (in the range of 100 μm from thesurface) included ferrite, bainite, martensite, residual austenite,cementite, and inclusions derived from alloy elements other than iron asin Examples 1 to 6 (FIG. 7 (microstructure) and FIG. 8 (inclusion)). Themicrostructure and inclusions in Examples 7 to 10 are the same as thosein Examples 1 to 6.

The microstructure and inclusions in Comparative Examples 6 to 11 arethe same as those in Comparative Examples 1 to 5. That is, a mixedstructure of bainite and martensite not including ferrite was observedin Comparative Examples 6 to 8, and a single phase of ferrite wasobserved in Comparative Examples 9 to 11. In Comparative Examples 6 to8, the cementite was hardly observed. In Comparative Examples 9 to 11,however, the cementite having very high number density greatly exceeding0.8 pieces/μm³ was observed.

This application is based upon and claims the benefit of priority of theprior Japanese Patent application No. 2013-099243, filed on May 9, 2013,the entire contents of which are incorporated herein by reference.

INDUSTRIAL APPLICABILITY

As described above, according to the invention, it is possible toprevent the delayed fracture occurring in the hot-sheared portionwithout increasing the shearing time or new steps during the hotshearing of the steel sheet. Accordingly, the invention has highapplicability in a steel sheet working technology industry.

1. A surface layer grain refining hot-shearing method comprising:heating and keeping a steel sheet having a carbon content of 0.15% ormore by mass in a temperature range of from Ac3 to 1400° C. toaustenitize the steel sheet; subsequently shearing the steel sheet in astate in which the steel sheet is placed on a die; and quenching byrapidly cooling the sheared steel sheet, wherein a start temperature ofthe shearing is set to be a temperature (° C.) obtained by adding atemperature of from 30° C. to 140° C. to a previously measured Ar3 ofthe steel sheet.
 2. A surface layer grain refining hot-shearing methodcomprising: heating and keeping a steel sheet having a carbon content of0.15% or more by mass in a temperature range of from Ac3 to 1400° C. toaustenitize the steel sheet; subsequently shearing the steel sheet in astate in which the steel sheet is placed on a die; and quenching byrapidly cooling the sheared steel sheet, wherein a start temperature ofthe shearing is set to be a temperature (° C.) obtained by adding avalue, which is calculated by multiplying an amount of equivalentplastic strain of a surface layer in a sheared portion by a coefficientfrom 40 to 60, to a previously measured Ar3 of the steel sheet.
 3. Thesurface layer grain refining hot-shearing method according to claim 2,wherein the amount of equivalent plastic strain of the surface layer inthe sheared portion is calculated as an average value of an amount ofequivalent plastic strain of a region in a range of from 5% to 20% of athickness of the steel sheet from a shear plane of the sheared portionto an inside of the steel sheet in a normal direction of the shear planeand in a range of from 20% to 50% of the thickness of the steel sheet ina thickness direction of the steel sheet from a bottom on a burr side ofthe sheared portion.
 4. The surface layer grain refining hot-shearingmethod according to claim 2, wherein the amount of equivalent plasticstrain of the surface layer in the sheared portion is calculated by anumerical simulation that is performed based on a stress-strain diagramat a steel sheet temperature of from 500° C. to 800° C.
 5. The surfacelayer grain refilling hot-shearing method according to claim 2, whereinthe amount of equivalent plastic strain of the surface layer in thesheared portion is calculated based on a Mises yield functionrepresented by the following Formula (1) $\begin{matrix}{{d\; {\overset{\_}{ɛ}}_{P}} = {\sqrt{\frac{2}{3}\left( {{d\; ɛ_{xx}^{2}} + {d\; ɛ_{yy}^{2}} + {d\; ɛ_{zz}^{2}} + {2d\; ɛ_{xy}^{2}} + {2d\; ɛ_{yz}^{2}} + {2d\; ɛ_{zx}^{2}}} \right)}.}} & (1)\end{matrix}$
 6. The surface layer grain refining hot-shearing methodaccording to claim 1, wherein the shearing of the steel sheet startswithin three seconds after the steel sheet comes in contact with thedie.
 7. The surface layer grain refining hot-shearing method accordingto claim 1, wherein the rapid cooling is performed when the steel sheetcomes in contact with the die.
 8. The surface layer grain refininghot-shearing method according to claim 1, wherein the rapid cooling isperformed when water jetting from a puncture formed in a contactingportion of the steel sheet with the die passes through a groove providedin the contacting portion of the steel sheet.
 9. The surface layer grainrefining hot-shearing method according to claim 1, wherein press formingnot accompanying fracture of the steel sheet is performed between theheating and the shearing of the steel sheet.
 10. A workpiece obtained bysurface layer grain refining hot-shearing, comprising: a steel sheethaving a carbon content of 0.15% or more by mass, a surface layer of asheared portion of a steel sheet having a carbon content of 0.15% ormore by mass including a ferrite phase and a remainder, the surfacelayer being defined as a range up to 100 μm inside of the steel sheet ina normal direction of a shear plane from a fracture plane of the shearedportion, wherein the remainder includes at least one phase of a bainitephase, a martensite phase, or a residual austenite phase which has acrystal grain diameter of 3 μm or less, and includes cementite andinevitably generated inclusions, wherein the ferrite phase has anaverage grain size of 3 μm or less, wherein the surface layer contains5% or more grains by number having an aspect ratio of 3 or more, andwherein a region out of the range of 100 μm includes: martensite andinevitably generated inclusions; or bainite, martensite, and inevitablygenerated inclusions.
 11. The workpiece obtained by surface layer grainrefining hot-shearing according to claim 10, wherein, in the surfacelayer, the cementite has a number density of 0.8 pieces/μm³ or less andthe cementite has a maximum length of 3 μm or less.
 12. The workpieceobtained by surface layer grain refining hot-shearing according to claim10, wherein a total area ratio of the bainite phase, the martensitephase, and the residual austenite phase, which are measured by anelectron-beam backscattering diffraction (EBSD) method, is from 10% to50% in the surface layer.
 13. A workpiece obtained by surface layergrain refining hot-shearing, the workpiece produced by: heating andkeeping a steel sheet having a carbon content of 0.15% or more by massin a temperature range of from Ac3 to 1400° C. to austenitize the steelsheet; subsequently shearing the steel sheet in a state in which thesteel sheet is placed on a die; and quenching by rapidly cooling thesheared steel sheet, wherein a start temperature of the shearing is setto be a temperature (° C.) obtained by adding a temperature of from 30°C. to 140° C. to a previously measured Ar3 of the steel sheet.
 14. Aworkpiece obtained by surface layer grain refining hot-shearing, theworkpiece produced by heating and keeping a steel sheet having a carboncontent of 0.15% or more by mass in a temperature range of from Ac3 to1400° C. to austenitize the steel sheet; subsequently shearing the steelsheet in a state in which the steel sheet is placed on a die; andquenching by rapidly cooling the sheared steel sheet, and a starttemperature of the shearing is set to be a temperature (° C.) obtainedby adding a value, which is calculated by multiplying an amount ofequivalent plastic strain of a surface layer in a sheared portion by acoefficient from 40 to 60, to a previously measured Ar3 of the steelsheet.
 15. The surface layer grain refining hot-shearing methodaccording to claim 2, wherein the shearing of the steel sheet startswithin three seconds after the steel sheet comes in contact with thedie.
 16. The surface layer grain refining hot-shearing method accordingto claim 2, wherein the rapid cooling is performed when the steel sheetcomes in contact with the die.
 17. The surface layer grain refininghot-shearing method according to claim 2, wherein the rapid cooling isperformed when water jetting from a puncture formed in a contactingportion of the steel sheet with the die passes through a groove providedin the contacting portion of the steel sheet.
 18. The surface layergrain refining hot-shearing method according to claim 2, wherein pressforming not accompanying fracture of the steel sheet is performedbetween the heating and the shearing of the steel sheet.